1. Field of the Invention
The present invention relates to an exhaust gas recirculation control device wherein a control is performed in which a part of exhaust gas of an internal combustion engine is recirculated back to an intake air pipe of the internal combustion engine, and its failure diagnosis device.
2. Discussion of Background
Formerly, an exhaust gas recirculation control device (hereinafter, EGR control device) which performs a control of exhaust gas recirculation, (hereinafter, EGR) as a means decreasing NO.sub.x in the exhaust gas of the internal combustion engine, is widely utilized. This EGR control device controls EGR by an exhaust pressure control system using BPT (Back Pressure Transducer) valve.
That is to say, a passage area of an EGR valve (recirculation valve) is controlled by a BPT valve so that a flow quantity of EGR becomes a predetermined value. Furthermore, in a system using a VVT (Venturi Vacuum Transducer) and a system in which the EGR control pressure is controlled by using a solenoid, similar to the system using the BPT valve, the passage area of the EGR valve is controlled.
As a means of diagnosing a failure of such EGR control device, conventionally, a device is proposed which is shown in Japanese Unexamined Patent Publication No. 256546/1985. In this device, a pressure difference between pressure values of a place adjacent to an exit of the EGR valve and an intake pipe, a clogging of the EGR valve is determined from the condition of the pressure difference.
Since the above-mentioned conventional EGR control device is composed using a VVT valve or the like, an exhaust gas recirculation quantity, that is, an EGR flow quantity can not be detected directly. As a result, when the EGR flow quantity is increased due to a deterioration of the BPT valve or the like, worsening of the drivability is caused. When the EGR flow quantity is decreased, the temperature of the engine is elevated and the NO.sub.x composition in the exhaust gas is increased. When the internal combustion engine is started up by making an ignition key switch ON, the actual EGR ratio, (first exhaust gas recirculation ratio) does not agree with a target EGR ratio (second exhaust gas recirculation ratio) for the time being by an influence of a timewise change or the like, the worsening of exhaust gas is caused for that period. Furthermore, when this device is in abnormal state, due to a deterioration of parts in the EGR control device, the abnormality of the device is difficult to be detected since the EGR flow quantity can not be detected directly.
On the other hand, in the above-mentioned conventional failure diagnose device of the EGR control device, although the clogging of the EGR valve can be detected to some degree, a failure diagnosis in case that the EGR flow quantity is increased due to a deterioration of BPT or the like, can not be performed accurately.